The present invention relates to the monitoring of vital signs in patients and is particularly useful for monitoring respiration in combination with other vital signs.
Apnea, the cessation of breathing or respiration, is associated with sudden infant death syndrome. Even if death does not result from an episode of apnea, irreversible brain damage may be caused. Sleep apnea is also a problem in adult patients and can have similarly severe consequences.
Accordingly, numerous attempts have been made to provide reliable and practical respiration monitors for clinical and home use. However, all such monitors have suffered from either reliability or practicality problems, or both. Consequently, the art has long felt the need to provide a patient monitor which overcomes these disadvantages.
Although it is possible to reliably monitor respiration by directly measuring air flow to and from a patient's lungs with a use of a flow meter coupled with a mask over the patient's nose and mouth, this technique is cumbersome and it is especially impractical for home use.
Since respiration monitoring of infants is often accompanied by EKG monitoring, a widely used respiration monitoring technique utilizes a measurement of thoracic impedance as it varies with the expansion and contraction of the patient's chest since this signal is easily acquired from the EKG electrodes. However, this method produces a signal having an indigenously low and highly variable signal-to-noise ratio when it is separated from the overriding EKG signal. Accordingly, the thoracic impedance signal is subject to interference by noise and the change in impedance caused by the vascular component resulting from the heart's pumping action. This problem becomes particularly troublesome when the patient's respiratory efforts diminish. Under these circumstances, signal processing circuits are prone to begin detecting the vascular component as valid respiration signals. When, for example, an infant patient's heart rate descends to between 40 and 50 beats per minute, the heartbeat's vascular component (that is, impedance change) across the patient's chest which passes through the thoracic impedance respiration filters appears identical to, and therefore intrinsically inseparable from, a valid impedance respiration waveform. Consequently, apnea can occur without being detectable by this method.
Most EKG and respiration monitors are coupled with the EKG electrodes directly by means of a cable. Although signal coupling techniques utilizing transformers or optocouplers have been adopted for reducing the potential for electric shock through the monitor itself, the mere fact that the cable provides a low resistance path to a patient's heart increases the possibility of electric shock. Further disadvantages include the possibility the patient may become entangled in the cable and the need to connect and disconnect the cable at various times.
An alternative approach to monitoring respiration utilizes a silastic strain gage containing metallic mercury which is arranged about the patient's torso and provides a variable resistance as it expands and contracts in response to changes in the size of the patient's torso with respiration. While this technique is usable in a laboratory environment, it is impractical for clinical or home use since the strain gage is relatively expensive and has a limited usable life. In addition, the strain gage must be applied to the patient's body in a state of tension which can be altered inadvertently as the patient moves. Other forms of strain gage type respiration transducers, such as piezoelectric transducers, share similar problems and are generally less sensitive and reliable. Body motion sensors have also been proposed as a means of monitoring respiration, although these devices are subject to motion artifact.
It is also possible to monitor respiration with the use of a pair of electrical inductors positioned on the torso of a patient and exhibiting a mutual inductance which varies as the patient's torso expands and contracts in the course of breathing. U.S. Pat. No. 4,494,553 to Sciarra, et al. shows a vital signs monitor which utilizes a belt positioned around a patient's chest and having a pair of coils mounted therein and spaced from each other so that, as the patient's chest moves on breathing, the coils move with respect to each other causing a change in the mutual or relative inductance of these coils. The coils are connected to a loop oscillator in a patient unit carried by the belt which forms an output signal whose frequency changes based on changes in the inductance of the coils due to the movement of the chest. Also included in the belt is a frequency modulation transmitter for transmitting respiration and cardiac signals to a nearby monitor.
However, since the inductors are spaced apart they have a relatively low mutual inductance so that breathing activity induces relatively low level changes therein. In addition, their mutual inductance is susceptible to change as a result of motion artifact. This susceptibility requires that the shape or geometry of the inductors must be fixed so that distortion of the respiration signal due to bending or other geometric distortion of the inductors is prevented. Accordingly, relatively expensive devices, such as printed circuit board inductors must be utilized. However, an optimum operating frequency range for inductive respiration monitoring is typically between about 200 and 350 kHz. This parameter, along with the fact that printed circuit board inductors permit only a limited number of turns on a single printed circuit board, require the use of multiple stacked printed circuit board inductors for monitoring respiration.
Traditional medical telemetry techniques utilize crystal controlled VHF transmitters to provide a high degree of frequency stability. However, prior art crystal controlled transmitters have required the use of high stability biasing schemes utilizing either zener diodes or voltage regulators to ensure that the transmitter will be operable. Such biasing schemes, in turn, require the use of relatively high voltages in the order of five volts or more to provide the necessary current for operating these devices. Consequently, prior art transmitters require relatively large power supplies.
Prior art medical telemetry systems also utilize either digital encoding and pulse modulation or full bandwidth analog transmission and so must operate within a relatively wide bandwidth in the order of .+-.20 kHz. If amplitude modulation is employed, relatively high modulation power is required, once again involving the use of a large power supply. However, if frequency modulation is instead utilized, it is difficult to carry this out with the use of a crystal controlled transmitter in the medical telemetry band of approximately 200 MHz. That is, to achieve a bandwidth of 20 kHz, it is necessary to add frequency multipliers to achieve the required deviation since crystals having a fundamental frequency of 200 MHz are not available. Moreover, frequency multiplication results in signal attenuation which requires the use of active amplification circuitry which consumes additional power. Accordingly amplitude modulation schemes typically are employed, thus requiring relatively high power and wide bandwidth operation, resulting in the possibility of signal interference from other transmissions.
The foregoing results in a large and fairly cumbersome patient unit which does not readily lend itself to a disposable-type device and the art has, therefore, employed units which are designed to be reusable. This approach is impractical of infant patient monitoring due to the relatively large size of the patient worn unit. Moreover, its clinical use for monitoring neonates is also impractical since the equipment typically becomes contaminated with body fluids and medications in this environment, thus requiring either disposability or sterilizability. Disposability is, of course, preferred since sterilization is inconvenient.
The patient unit of U.S. Pat. No. 4,494,553 aligns the cardiac transducer with the respiration transducing inductors longitudinally on the patient's thorax in order to simultaneously monitor cardiac function as well as respiration represented by the expansion and contraction of the chest cavity. Accordingly, this device is not well adapted for monitoring respiration in infants who are principally belly breathers.
It is often desirable to provide the ability to make tidal volume respiration measurements by simultaneously measuring size changes at predetermined longitudinally spaced apart positions on a patient's torso in order to detect central or obstructive apnea. This ability is not provided by the device of U.S. Pat. No. 4,494,553 which is only adapted for measuring size changes at one position. In the prior art, one tidal volume measurement device utilizes a flexible net which is arranged about the patient's entire upper torso so that the flexible net can expand and contract therewith. A pair of longitudinally spaced, single coil inductors are stitched into the net in a sawtooth configuration, so that the inductance represented by each single coil inductor varies with the expansion and contraction of the patient's torso thereunder. Each coil is coupled with a respective one of two oscillator circuits in order to vary the oscillation frequency of the corresponding circuit as the patient's torso expands and contracts. However, flexibility of the net also permits the longitudinal spacing of the inductors to change uncontrollably and undesireably.